1. Field of the Invention
This invention relates to the recovery of hydrocarbons from subterrenean oil shale formations. A method is provided for the in situ heating of the subterranean oil shale formation using two horizontal, vertically spaced metallic electrodes formed from cooling molten metal in fractures of the oil shale formation. More particularly, the invention relates to the recovery of hydrocarbons from the formation by drilling a bore hole, fracturing the oil shale formation near the upper and lower boundaries of the formation, injecting molten metal into the relatively horizontal fractures, allowing the metal to cool to form vertically spaced metallic electrodes, providing a radio frequency transmission line or coaxial cable between the electrodes, and inducing unterminated standing waves in the upper and lower metallic electrodes and in the oil shale formation therebetween by means of a radio frequency generator.
2. Brief Description of the Prior Art
Subterannean oil shale formations contain relatively large amounts of valuable hydrocarbons, but the large scale commercial recovery of these hydrocarbons has been hindered by economical and environmental constraints. Deep mining and strip mining techniques such as those used to mine coal have proved to be an inefficient method of recovering the hydrocarbons due to the large amount of bulk shale which must be extracted to produce the hydrocarbons. Additionally, these techniques negatively affect the environment and a large amount of unusable rock byproduct must be disposed of.
To avoid these difficulties numerous in situ processes of heating the oil shale within the subterranean formation have been proposed. Application of heat to the oil shale rock increases the porosity and permeability of the oil shale. Upon pyrolysis, the oil shale yields a condensable liquid which can be refined into hydrocarbons including petroleum products.
Processes by which super-heated steam or hot liquid had been injected into the oil shale formation have all proved to be commercially unacceptable since an effective flow of kerogens from the formation could not be readily achieved. These techniques also do not allow for the uniform heating of the oil shale formation due to the low thermal conductivity of the rock.
Other techniques have also been proposed but these have met similar disadvantages. Partial combustion of the hydrocarbons within the subterranean oil shale formation is generally inefficient, environmentally damaging, and difficult to control adequately. Infusion of heat energy to the oil shale formation by electrical induction heating likewise fails to provide a commercially adequate recovery of hydrocarbons due to the limited thermal and electrical conductivity of the bulk formations.
It has been proposed that the uniform heating of the rock formation can be achieved by using ratio frequency (R.F.) electrical energy which corresponds to the dielectric absorption characteristics of the rock formation. An example of such techniques is described in U.S. Pat. No. 4,140,180 and 4,144,935 in which a plurality of vertical conductors are inserted into the rock formation and bound a particular volume of the formation. A frequency of electrical excitation is selected to attain a relatively uniform heating of the rock formation.
Similarly, U.S. Pat. No. 4,135,579 and 4,196,329 describe a method and apparatus by which an alternating electrical field is produced between vertical electrode structures inserted into the subterranean formation. Temperature gradients within the rock formation result from the electrical field so as to fracture the rock body. Modification of this technique is described in U.S. Pat. No. 4,140,179 in which the amount of liquid water in the rock formation is reduced prior to supplying the electric field in order to decrease the temperature needed for pyrolysis of the hydrocarbons.
The difficulty with the above-described techniques using R.F. energy to heat the formation is the necessity of implanting an electrode within the subterranean rock formation at a precise distance. The electrodes in these processes are described to be pipes, transmission lines, conductive plates, and variations thereof. Such an insertion and the proper spacing thereof has proved to be difficult to achieve, time consuming, costly, and inefficient.
There have been some suggestions of forming fractures directly within the rock formation and applying heat to the formation in order to recover hydrocarbons from the formation. U.S. Pat. No. 4,030,549 discloses the injection of the reactive slurry comprising finely divided aluminum and a reactive metal oxide into a fracture and the subsequent ignition of the slurry by a thermite reaction to form a molten metal in the fracture system. U.S. Pat. No. 3,149,672 suggests propping fractures in the rock formation with particles of an electrical conductor, such as aluminum, iron or copper spheres, and connecting the fractures with a source of electric current. However, these methods lack the ease and efficiency which results from directly injecting molten metal into the fracture without the need for a subsequent chemical reaction within the fracture, or without uncertainty in obtaining suitable electrical conduction.
It is an object of the present invention to provide an in situ pyrolysis process of heating hydrocarbons contained in subterranean oil shale formations, in such a manner that relatively large amounts of hydrocarbons are recovered.
A further object of the present invention is the provision of a method by which relatively horizontal metallic electrodes vertically spaced apart are formed in the subterranean formation between which unterminated standing waves induced by a radio frequency generator can be passed.
It is an object of the present invention to recover vaporized hydrocarbons from the in situ heating of a subterranean oil shale formation in an economical and efficient manner which may require only a single bore hole, with a minimum of adverse environmental impacts.
Further objects and advantage of this invention will become apparent in study of the following portion of the specifications, claims, and the attached drawings.